This is correct; until someone on those remote instances searches (or maybe subscribes?) then the c/ultralight won’t be federated to those remote instances. You should use a larger community as your test case, if you are trying to verify federation, or take is a sign that your (very niche?) ultralight community just hasn’t got many subscribers yet.
Why would you create an account on every server? You can access any community on any server that your server is federated with.